1. Field of the Invention
The present invention relates to an optical head device and an optical information recording or reproducing device for performing recording or reproduction to/from a plurality of types of optical recording medium and, more specifically, to an optical head device and an optical information recording or reproducing device, which can achieve a stable track error signal by small size and also exhibits high efficiency. In the followings, light of 400 nm band wavelength for HD DVD is referred to as light PH, light of 650 nm band wavelength for DVD as light PD, and light of 780 nm band wavelength for CD as light PC.
2. Description of the Related Art
Optical head devices which perform recording and reproduction to/from a plurality of types of optical recording medium of different standards such as DVD (Digital Versatile Disk) and CD (Compact Disk) have been put into practical use. However, the recording or reproducing characteristics for the optical recording medium of a specific standard are guaranteed only for a specific wavelength. That is, the recording or reproducing characteristics for the optical recording medium of DVD standard are guaranteed only for the light PD, and the recording or reproducing characteristics for the optical recording medium of CD standard are guaranteed only for the light PC.
Therefore, in general, the optical head device which performs recording and reproduction to/from a plurality of types of optical recording medium of different standards comprises a plurality of numbers of light sources being mounted for outputting the light of wavelengths corresponding to each standard. For example, the optical head device which performs recording and reproduction to/from both the optical recording medium of DVD standard and the optical recording medium of CD standard is provided with a light source for emitting the light PD and a light source for emitting the light PC.
In general, each of these two light sources is housed in a separate package. However, a double-wavelength light source in which two light sources are housed in a common package has also been put into practical use. By using such double-wavelength light source for the optical system of the optical head device, it becomes unnecessary to provide a photosynthesizing device for synthesizing light emitted from two light sources housed in separate packages. Thus, size of the optical system of the optical head device can be reduced.
Known as a method for detecting a track error signal in the optical head device is a differential push-pull method. With the differential push-pull method, there is no offset generated in the track error signal even if an objective lens of the optical head device shifts in a radial direction of the optical recording medium thereby enabling to attain a stable track error signal. This detection method can be achieved through dividing the emitted light from the light source into 0th-order light as a main beam and ±1st-order diffracted light as sub-beams by a diffractive optical element provided between the light source and the objective lens of the optical head device. These three beams of light are reflected by the optical recording medium and separately received by a photodetector of the optical head device. In the differential push-pull method, the track error signal is attained by finding a difference between a push-pull signal of the main beam and push-pull signals of the sub-beams received by the photodetector. The ratio of light quantities of the main beam and the sub-beams is determined by the groove depth of a diffraction grating of the diffractive optical element. The space between the main beam and the sub-beams on the optical recording medium is determined by a groove cycle of the diffraction grating of the diffractive optical element.
Now, let's look into the case of detecting the track error signal using the above-described diffractive optical element for each of the optical recording medium of the DVD standard and the optical recording medium of the CD standard in the optical head device which performs recording and reproduction to both the optical recording medium of the DVD standard and the optical recording medium of the CD standard.
When using two light sources for DVD and CD being housed in the separate packages for the optical system of the optical head device, in general, a diffractive optical element for DVD is disposed on an exclusive optical path of the light PD and a diffractive optical element for CD is disposed on an exclusive optical path of the light PC. By individually setting the groove depth and the cycles of the diffraction gratings of the respective diffractive optical elements, the ratio of the light quantities of the main beam and the sub-beams and the space therebetween on the optical recording medium can be set individually for the light PD and the light PC.
In the meantime, when using the double-wavelength light source for DVD and CD housed in a common package for the optical system of the optical head device, the diffractive optical element compatible for double wavelengths of DVD and CD has to be disposed on a common optical path of the light PD and the light PC. Thus, for individually setting the ratio of the light quantities of the main beam and the sub-beams and the space therebetween on the optical recording medium for the light PD and the light PC, some contrivances are necessary in the diffractive optical element which is compatible for double wavelengths.
Japanese Patent Unexamined Publication No. 2001-290017 discloses such an optical head device which uses the double-wavelength light source and the diffractive optical element compatible for double wavelengths as described above. FIG. 1 shows the optical head device disclosed in Japanese Patent Unexamined Publication No. 2001-290017. A semiconductor laser 1a is obtained by housing a semiconductor laser for emitting the light PD and a semiconductor laser for emitting the light PC in a common package.
The light PD emitted from the semiconductor laser 1a is divided into three light beams by a diffractive optical element 35 which are 0th-order light as a main beam and ±1st-order diffracted lights as sub-beams. A part of these light beams transmits through the non-polarizing beam splitter 36, collimated by a collimator lens 2, and is focused onto a disk 7 as an optical recording medium of the DVD standard by an objective lens 6. The three light beams reflected by the disk 7 transmits through the objective lens 6 and the collimator lens 2 in the reverse direction and a part of which is reflected by the beam splitter 36 and received by a photodetector 10a. 
The light PC emitted from the semiconductor laser 1a is divided into three light beams by a diffractive optical element 35 which are 0th-order light as a main beam and ±1st-order diffracted lights as sub-beams. A part of these light beams transmits through a non-polarizing beam splitter 36, collimated by a collimator lens 2, and is focused onto the disk 7 as an optical recording medium of the CD standard by the objective lens 6. The three light beams reflected by the disk 7 transmits through the objective lens 6 and the collimator lens 2 in the reverse direction and a part of which is reflected by the beam splitter 36 and received by the photodetector 10a. 
FIG. 2 is a cross section of the diffractive optical element 35. The diffractive optical element 35 has a configuration in which: a diffraction grating 38a having a birefringent characteristic is formed on a substrate 37a; a diffraction grating 38b having a birefringent characteristic is formed on a wave plate 41; a filler 39 is filled in between the diffraction grating 38a and the diffraction grating 38b; and the wave plate 41 and the substrate 37a are bonded by an adhesive 40. Here, the linearly polarized light whose polarization direction is parallel to the grooves of the diffraction gratings 38a, 38b is referred to as TE-polarized light and the linearly polarized light whose polarization direction is vertical to the grooves of the diffraction gratings 38a, 38b is referred to as TM-polarized light. At this time, the refractive index of the diffraction grating 38a is equal to the refractive index of the filler 39 for the TE-polarized light and is different from the refractive index of the filler 39 for the TM-polarized light. Further, the refractive index of the diffraction grating 38b is different from the refractive index of the filler 39 for the TE-polarized light and is equal to the refractive index of the filler 39 for the TM-polarized light. That is, the diffraction grating 38a functions as the diffraction grating only for the TM-polarized light and the diffraction grating 38b functions as the diffraction grating only for the TE-polarized light.
As shown in FIG. 2(a), the light PD makes incidence to the diffraction gratings 38a, 38b as the TE-polarized light. Thus, the light PD almost entirely transmits through the diffraction grating 38a and is divided into three light beams, i.e. 0th-order light as the main beam and the ±1st-order diffracted light as the sub-beams by the diffraction grating 38b. The ratio of light quantities of the main beam and the sub-beams is determined by the groove depth of the diffraction grating 38b and the space between the main beam and the sub-beams on the optical recording medium is determined by the groove cycle of the diffraction grating 38b. 
As shown in FIG. 2(b), the light PC makes an incident to the diffraction gratings 38a, 38b as the TM-polarized light. Thus, the light PC is divided into three light beams, i.e. 0th-order light as the main beam and the ±1st-order diffracted light as the sub-beams by the diffraction grating 38a and almost entirely transmits through the diffraction grating 38b. The ratio of light quantities of the main beam and the sub-beams is determined by the groove depth of the diffraction grating 38a and the space between the main beam and the sub-beams on the optical recording medium is determined by the groove cycle of the diffraction grating 38a. 
In the manner as described above, the ratio of the light quantities of the main beam and the sub-beams and the space therebetween on the optical recording medium can be individually set for the light PD and the light PC. The wave plate 41 functions as a quarter-wave plate for the light PD. Thus, in FIG. 2(a), the light emitted from the diffractive optical element 35 becomes a circularly polarized light. Further, in FIG. 2 (b), the light emitted from the diffractive optical element 35 becomes an elliptically polarized light in general.
In an optical head device shown in FIG. 1, for example, for obtaining the maximum product of the efficiency of the inward light emitted from the semiconductor laser 1a towards the disk 7 through the beam splitter 36 and the efficiency of the outward light towards the photodetector 10a reflected by the disk 7 and the beam splitter 36, the transmittivity and the reflectivity of the beam splitter 36 are both to be 50%. That is, when the non-polarizing beam splitter 36 is used as a light separating element for separating the inward light and the outward light, there generates 50% loss in the light quantities generated both in the inward and outward light. In order to suppress the loss of the light quantity in the inward and outward light, generally used as the light separating element is a combination of a polarizing beam splitter and a quarter-wave plate.
However, even if the beam splitter 36 of the optical head device shown in FIG. 1 is replaced with the combination of the polarizing beam splitter and the quarter-wave plate, loss in the light quantity of the outward light cannot be suppressed. The reason is that the light PD for DVD makes incidence to the polarizing beam splitter as a circularly polarized light so that only about 50% of the light is transmitted, and the light PC for the CD generally makes incidence to the polarizing beam splitter as an elliptically polarized light so that the light is not transmitted by 100%.
As described above, in the optical head device disclosed in Japanese Patent Unexamined Publication No. 2001-290017 and the optical head device in which a part of the optical system is modified, the optical system can be reduced in size by using the double-wavelength light source for the optical system, and the ratio of the light quantities of the main beam and the sub-beams and the space therebetween on the optical recording medium can be individually set for the light of the respective wavelength. Thus, it is possible to attain a stable track error signal by the differential push-pull method. However, loss of the light quantity in the inward and outward light cannot be suppressed so that the efficiency is low.